30  Chemical  Observations  on  Tartar  Emetic.    {Am  j^Jeg*"11, 
and  gradually  nearly  the  whole  of  the  antimony  falls  as  a  white  pre- 
cipitate of  antimonious  hydrate. 
NaHC03+Sb.OK.C4H406+H20= 
KNaC4H406+Sb(OH)3+C02. 
If  a  solution  of  iodine  is  added  to  the  liquid  before  the  precipitation 
has  commenced  it  is  immediately  decolorized,  and  a  sharp  termination 
of  the  reaction  is  observed.  If,  however,  the  solution  of  iodine  is  not 
added  until  precipitation  has  commenced,  then  wholly  incorrect  re- 
sults will  be  obtained,  since  the  precipitated  hydrate  is  hardly  attacked 
by  the  iodine.  To  illustrate  the  importance  of  accomplishing  the  re- 
action before  precipitation  has  set  in,  an  experiment  may  be  quoted  in 
which  two  solutions  of  tartar  emetic  containing  precisely  the  same 
quantity  of  the  salt  were  mixed  with  an  equal  quantity  of  sodium  bi- 
carbonate. In  the  one  solution,  titration  with  iodine  was  immediately 
performed,  when  it  was  found  that  18  cubic  centimetres  of  the  deci- 
normal  solution  of  iodine  were  consumed.  In  the  other  solution  titra- 
tion was  not  commenced  until  after  the  lapse  of  some  hours,  by  which 
time  nearly  the  whole  of  the  antimony  had  been  precipitated.  The 
mixture  was  now  found  to  require  less  than  half  a  cubic  centimetre  of 
the  solution  of  iodine  to  complete  the  oxidation  of  the  small  quantity 
of  antimony  compound  which  had  not  been  precipitated.  For  this 
reason,  a  solution  of  bicarbonate  should  be  added  to  the  dissolved  tar- 
tar emetic  immediately  before  the  titration  is  conducted.  In  the  ex- 
periments recorded  below  from  10-20  cubic  centimetres  of  a  five  per 
cent,  solution  of  sodium  bicarbonate  were  employed,  and  the  dilution 
of  the  solution  was  varied.  We  have  not  found  a  large  quantity, 
either  of  sodium  bicarbonate  or  of  water,  to  affect  the  chemical  change 
to  any  appreciable  extent.  In  practice,  it  will  be  found  that  0*2-0"3 
gram  of  tartar  emetic  is  a  convenient  quantity  to  take  for  each  estima- 
tion. 
Tartar  emetic  taken.  Dilution.  Tartar  emetic  found. 
0-1    gram  80  cc.  0100 
01      "  100  "  0100 
'    0*2      "  20  "  0  200 
0-2      "  100  «  0-200 
0-4937  "  100  "  0-4926 
0-5472  "  100  "  0-5465 
These  results  conclusively  prove  that  the  reaction  occurring  between 
solutions  of  tartar  emetic  and  iodine  in  the  presence  of  sodium  bicarb- 
